Predator responses to prey population dynamics: an empirical analysis based on lake trout growth rates

Abstract

Lake trout (Salvelinus namaycush) growth rates were lower in Lake Superior (with the lowest prey density) than in Lakes Michigan and Ontario. In contrast, consumption rates (estimated using bioenergetics models) of lake trout were similar in Lakes Superior, Michigan, and Ontario because the primary prey in Lake Superior, rainbow smelt (Osmerus mordax), have a lower energy content than the primary prey in Lakes Michigan and Ontario, alewife (Alosa pseudoharengus). Estimated consumption rates did not respond to three-to five-fold changes in prey density within lakes and were similar across a 100-fold difference in prey density among lakes. These results demonstrate that lake trout are able to sustain high predation rates at low prey densities and a type II functional response fitted to these data rises very steeply at low prey densities. Two factors are implicated: the large search volume of these predators and disparity between the average density estimates of prey stock assessment and the spatial and temporal scaling appropriate for understanding predator–prey interactions. If prey are highly aggregated or predators can search large areas, then a decrease in whole-lake prey abundance will not necessarily result in a lower predation rate until prey populations are severely reduced.